About Invasive Species
What are their impacts?
Biodiversity reduction is one of the most alarming impacts of the translocation of invasive species. One of the simplest, most well-known and devastating examples of a simple prey-predator association of an invasive species introduction was that of the introduction of the Nile perch (Lates_niloticus) into Lake Victoria by authorities in the 1960s. This led to the disappearance of up to 200 native cichlid fish species, described by some scientists as “the greatest single paroxysm of extinction ever recorded”.
More complicated associations occur when food webs are disrupted. For example the Leidy’s comb jelly (Mnemiopsis_leidyi) caused a trophic-level disaster when it was introduced into the Black Sea in Europe from the Americas via ballast water. The jelly fish began foraging on zooplankton and as it had no natural predators this foraging in turn lead to the collapse of anchovy farms and other fisheries on the Black Sea.
Island ecosystems appear to be more vulnerable to invasions. Island ecosystems tend to have fewer species present and are less complex with distance from the continent; simpler systems are less resilient to new arrivals. Feral pigs (Sus_scrofa) are an appropriate example of the damage which can be caused to island ecosystems by invasive species. Having invaded pristine ecological sites in the Hawaiian islands the pigs are damaging native ecosystems by uprooting native plants and facilitating the spread of other introduced plant and bird species, including mosquitoes which carry avian diseases.
Invasive rats are some of the largest contributors to seabird extinction and population declines globally. Studies show that of the 3 species of invasive rats, black rat (Rattus _rattus) had the largest mean impact on seabirds followed by the Norway rat (R._norvegicus) and ship rat (R._exulans) (See the video clip below)
Pianosa Island -Tuscan Archipelago (July 2013) Scopoli's shearwater/ Cory's shearwater (Calonectris_diomedea) chick being attacked by a black rat
Project implemented with funding from the Tuscan Archipelago National Park and the Tuscany Regional Government.
Another example is the small Indian mongoose (Herpestes_auropunctatus/ Herpestes_javanicus) introduced to many islands worldwide for control of rats and snakes, mainly in tropical areas, and also to islands in the Adriatic Sea. Mongooses are diurnal generalist carnivores that thrive in human-altered habitats. Predation by mongoose has had severe impacts on native biodiversity leading to the decline and extirpation of native mammals, birds, reptiles, and amphibians. Analysis of collection records indicates that the loss or decline of 14 Neotropical skink species in the Caribbean islands can be attributed to predation by the small Indian mongoose. The ground-dwelling and diurnal habits of skinks have made them particularly susceptible to mongoose predation.
Whole ecosystem processes can come under threat from invasive species. First recognized in North America in 1889, cheat grass (Bromus_tectorum) a native of Eurasia produces a litter that is slow to decompose and has drastically increased the frequency of fires in arid rangeland systems to a nearly annual cycle. This has caused an almost complete loss of native woody species over large areas. Due to lack of cover and forage, songbirds and rodents abandon areas dominated by cheat grass.
Invasive species come with a high monetary cost. It is estimated that damage and control cost of invasive species in the USA alone amounts to more than $138 billion annually. In addition, economic losses can occur through loss of recreational and tourism revenues. While monetary costs may be estimated, it is more difficult and perhaps inappropriate to put a price tag on the lose of native biodiversity and the negative impacts to native ecosystems.
The Global Invasive Species Database (GISD) uses the following categories of negative impacts of invasive species on native biodiversity: predation, competition, disease transmission, fouling, herbivory, human nuisance, interaction with other invasive species, physical disturbance, threat to endangered species and hybridisation with native species. The GISD categories also include impacts related to the disruptions of the function of native communities and ecosystems, including: habitat alteration, ecosystem change, modification of: hygrology, fire regime, natural benthic communities, nutrient regime and/or successional patterns.
The list of '100 of the World's Worst Invasive Alien Species' illustrates the incredible variety of species that have the ability, not just to travel in ingenious ways, but also to establish, thrive and dominate in new places. Today, alien invasion is second only to habitat loss as a cause of species endangerment and extinction.
Click here to view images and follow links to species profiles of '100 of the World's Worst Invasive Alien Species' on the Global Invasive Species Database.